1. Field of the Invention
The present invention relates to thermoplastic fitting electric welding methods and apparatus, and more particularly, but not by way of limitation, to methods and apparatus for electrically heat welding thermoplastic fittings having electric resistance heating elements disposed therein to other plastic members such as plastic pipe joints.
2. Description of the Prior Art
Electric heat weldable fittings formed of thermoplastic material have been developed and used heretofore. Such fittings generally include an electric resistance heating coil or element positioned adjacent the inside surfaces of the fitting which are to be welded to one or more other thermoplastic members such as plastic pipe sections. The electric resistance heating element is usually a coil of resistance wire disposed in the thermoplastic material of the fitting and is connected to electric contacts which are attached to an outside surface of the fitting. Examples of such electric heat weldable thermoplastic fittings are described in U.S. Pat. Nos. 4,147,926, issued Apr. 3, 1979, and 4,349,219, issued Sep. 14, 1982.
In welding the fitting to other plastic members positioned adjacent thereto, a source of electric power is connected to the contacts, such as by electric cable, and electric power is supplied to the resistance heating element of the fitting. The heating element heats the fitting and the adjacent thermoplastic members to temperatures which cause the thermoplastic materials from which the fitting and adjacent members are made to melt whereby they become fused or welded together.
The quality of the weld which results is primarily dependent upon the correct quantity of electric power being supplied to the heating element of the fitting. If too little electric power is supplied, too little heating takes place and an inadequate low strength weld results. If too much electric power is supplied, the fitting and plastic members to which the fitting is welded can be deformed and overheated whereby a good weld does not result. Other factors that affect the quality of the weld produced include heating element abnormalities, e.g., short circuits, poor fitting alignment, poor contact between surfaces to be welded, etc.
Various types of control and electric power generating apparatus have been utilized for supplying the electric power to electric heat weldable thermoplastic fittings. Initially, such apparatus were manually controlled by an operator and the quantity of electric power supplied to the heating elements of the fittings was determined by visual observation of the fittings as they were welded. More recently, control apparatus have been developed whereby the operator manually programs the control apparatus to supply a predetermined quantity of electric power to the heating element of the fitting in accordance with the particular size of the fitting. Also, electric heat weldable fittings having heating coils and separate resistors disposed therein have recently been developed. The values of the resistors are chosen in accordance with the size of the fitting and the electric power to be supplied thereto. Electric power control apparatus that are utilized with such fittings are adapted to sense the values of the resistors and automatically supply a preset quantity of electric power to the heating coil in accordance therewith. Examples of such fittings and control apparatus are described in U.S. Pat. No. 4,486,650, issued Dec. 4, 1984.
While the fittings including separate resistors and the control apparatus which automatically supply the heating coils of the fittings with predetermined quantities of electric power have generally achieved good results, because each of the fittings must include one or more resistors in addition to the resistance heating element disposed therein, they are more expensive to produce than fittings with heating elements alone. In addition, the electric power control apparatus have not included satisfactory provision for preventing the burn-up of fittings and the fire hazard attendant thereto when fittings having shorted-out heating coils are encountered. Also, the predetermined quantities of electric power supplied the fittings have not always been accurate because the temperatures of the fittings have not been taken into account. None of the prior electric control apparatus have been capable of detecting and accounting for defects such as bad electrical connections, misaligned pipe or other similar factors affecting the welding current or temperature.
By the present invention, a method and apparatus for electrically heat welding thermoplastic fittings are provided wherein the fittings do not require separate resistors or other devices for indicating the size of fitting and electric power required. In accordance with the present invention, the size of fitting is determined by impedance and/or quality factor determinations and the entire welding process of each fitting is comparatively monitored to insure the making of a high quality weld.